gRFC A21 Service Config Error Handling

[yas...@google.com]

This is the discussion thread for the proposal at https://github.com/grpc/proposal/pull/100.

The proposal provides a way to treat errors in service configs.

If a service config (JSON formatted) received is badly formatted or  contains invalid values for known fields, the client should ignore that service config in its entirety. Unknown fields in the service config should be ignored. If there was a service config that the client was previously using, it shall continue using those parameters when communicating with the service. If there was no service config being used, then the client shall continue behaving as if it never received a service config update from the resolver. On the other hand, if it is a new client, then the client should fail to start, i.e., not act on the resolution result till a valid service config is received.

[yas...@google.com]

After offline discussions, a change has been made to the proposal. If a default service config was provided as a channel argument, then that config would be a valid service config to use if there was no valid resolved service config to use.